1. STAHL AND MESELSON’S WORK ON SEMICONSERVATIVE( 半保留 ) NATURE OF DNA REPLICATION

3. MATTHEW STANLEY MESELSON (1930-) Born in Denver, Colorado Studied chemistry at the University of Chicago Worked at the California Institute of Technology

4. FRANKLIN WILLIAM STAHL (1929-) Born in Boston. Received a B.A. from Harvard University in 1951. Work in the University of Rochester after graduated.

5. In 1954, Meselson met Franklin Stahl In 1958, they proved that DNA was replicated in a semi- conservative way.

6. The Meselson-Stahl experiment was an experiment to prove that DNA replication was semiconservative.DNA replicationsemiconservative Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strandDNA NitrogenNitrogen is a major constituent of DNA, the genetic material of all cells.genetic materialcells It is commonly found in the 14 N isotope, but it can also be found in the heavier 15 N isotope.isotope

7. E. coliE. coli were grown for several generations in a medium with 15 N. The DNA of the resuling cells had a higher density (was heavier). E. coli cells with only 15 N in their DNA were put back into a 14 N medium and were allowed to divide only once. DNA was then extracted from a cell and was compared to DNA from 14 N DNA and 15 N DNA. It was found to have exactly an intermediate density. This supported the idea of semiconservative replication. The DNA was intermediate in density because it had an all 15 N DNA strand and an all 14 N DNA strand. The all 15 N strand was one of the original strands in the original cell. The all 14 N strand was a newly synthesized strand.

8. DNA replication or DNA synthesis is the process of copying a double-stranded DNA strand, prior to cell divisionDNAcell division The two resulting double strands are identical, and each of them consists of one original and one newly synthesized strand. This is called semiconservative replication.semiconservative replication The process of replication consists of three steps, initiation, replication and termination

9. After the helicase unwinds the DNA, single-strand binding protein is used to hold the DNAsingle-strand binding protein RNA primase is then bound to the starting DNA site. Begin of replication, an enzyme called DNA polymerase binds ( 凝 固 )to the RNA primase, which indicates the starting point for the replication.DNA polymerase DNA polymerase can only synthesize new DNA The DNA polymerase can only travel on one side of the original strand without any interruption. Since the DNA replication on the lagging strand is not continuous, a new DNA polymerase has to be added each time as the helicase unwinds more DNA. The replicated DNA is fragmented.

10. (DNA) n + dNTP ↔ (DNA) n+1 + PP i

11. Semiconservative replication describes the method by which DNA is replicated in all known cells.DNA Conservative replication would leave the original, template DNA strands intact and would produce a copy composed of entirely new DNA base pairs. Dispersive replication would produce two copies of the DNA, both containing a mixture of old and new DNA base pairs. Semiconservative replication would produce two copies that each contained one of the original strands, and one entirely new copy. The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that the semiconservative model was correct decipher